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3648Re: [Altair Computer Club] RE: Later 8in drive disk buffer drive assignment switches

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  • mfeberhard
    Jan 15, 2014
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      My drives are both the older style, with jumpers instead of a switch. The drives are addressed exactly as the manual says: the first drive (0) is A-bar, B-bar, C-bar, D-bar. The second one is A, B-bar, C-bar, D-bar.

      Looking at the schematics, I bet you have a broken wire between the controller in the Altair and the Disk Buffer Board in the drive unit - use a meter and ohm out pin 14 on the I/O connector. If this signal is discontinuous, then it will behave exactly as you describe: drive 0 will be selected when you select A, B-bar, C-bar, D-bar.

      You can check the entire signal path: check for continuity from IC C pin 10 on the Disk Buffer Board to IC K3 pin 5 on Disk Controller Board #2. I bet this signal is not connected - either in the external wiring, or the wiring inside the Altair. My next bet would be that IC K3 has failed, and is stuck high on its output. (Next I'd check IC J3, but I doubt this is the problem.)

      I also have one of those newer buffer boards, though not installed in a drive box. When those double switches are in the position marked ON on the silkscreen, they connect the non-inverted path (for example A). When they are in the other position, the connect the inverted path (for example A-bar). So drive zero would be selected if all switches are in the 'off' position.
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